1. Field
This application generally relates to systems and methods for sensing ground contact by prosthetic and/or orthotic devices.
2. Description of the Related Art
Advent of computer-controlled lower-limb prosthetic devices has made available to the lower-limb amputee an unprecedented realm of performance. The extent of the lower-limb locomotion tasks associated with daily living activities often cannot be addressed with a single lower-limb prosthetic device behavior. While non-computer controlled devices are relying on mechanical linkages, or mechanisms, which properties can hardly be modified to change their behavior, computer-controlled devices are quite more flexible, and assuming sufficient control of their actuation mechanism, can sustain a very large realm of lower-limb joint behaviors. For computer controlled lower-limb prosthetic devices to be able to vary their behavior in an appropriate and efficient manner, devices can be equipped with a sensor set allowing the embedded processor and control scheme to extract relevant information from the environment in which they are evolving and apply the behavior changes accordingly.
One area where computer-controlled lower-limb prosthetics are showing improved performance over passive or non-computer controlled devices is in the management of the dual mechanical configuration of the lower-limb joints during gait related activities. As the lower-limb transitions from the aerial phase to the ground contacting phase, lower-limb joints are observed to drastically change their behavior, going from a low mechanical impedance state during the aerial phase to a high impedance state during the contacting phase. Proper management of this joint behavior modification significantly improves lower-limb prosthetic devices usability and performance from a user standpoint. Moreover, management of such a transition is based on the system's capacity to know with accuracy and reliability whether or not the prosthetic foot is in contact with the ground.
Development of sensors allowing for ground contact detection in a robust and reliable manner represents a significant aspect of computer-controlled lower-limb prosthetic devices. In fact, many technologies are available to perform such detection, but all present limitations. Load cells are commonly used to perform this task, but are bulky, heavy and need periodical calibration to ensure that their no-load reading remains constant. Accelerometers are compact and allow for monitoring foot strike shocks, but only provide a contextual measure in the sense that if a specific event is missed, you have to wait to the next one to know for sure in which configuration the prosthetic limb is. Piezoelectric sensors are also compact, but present limited performance as far as DC measurement capacity is concerned. Resistive ink sensors can also be used to measure occurrence of foot strike in prosthetic system, but these are known to be fragile and present only a limited life duration in typical field operating conditions.
Moreover, development of a robust sensor system for ground contact detection requires a system simple enough where a priori information and assumptions are required to draw a conclusion on the actual lower-limb mechanical configuration. Hence, benefits arise at a system level if the ground contact sensor design itself is able to reject external perturbation and provides a measure showing high correlation with foot strike and only limited correlation with other loads affecting the system during typical operation. More specifically, computer-controlled devices with active actuation are subject to perturbation created by the foot inertia, as the system attempts to control the hip, knee or ankle behavior.